THERMODYNAMIC PROPERTIES ThermodynamicsSpecificGravitySpecific gravity (S.G.) is a measure of the relative density of a substance as compared to thedensity of water at a standard temperature. Physicists use 39.2°F (4°C) as the standard, butengineers ordinarily use 60°F. In the International System of Units (SI Units), the density ofwater is 1.00 g/cm3 at the standard temperature. Therefore, the specific gravity (which isdimensionless) for a liquid has the same numerical value as its density in units of g/cm3. Sincethe density of a fluid varies with temperature, specific gravities must be determined and specifiedat particular temperatures.HumidityHumidity is the amount of moisture (water vapor) in the air. It can be expressed as absolutehumidity or relative humidity. Absolute humidity is the mass of water vapor divided by a unitvolume of air (grams of water/cm3 of air). Relative humidity is the amount of water vaporpresent in the air divided by the maximum amount that the air could contain at that temperature.Relative humidity is expressed as a percentage. The relative humidity is 100% if the air issaturated with water vapor and 0% if no water vapor is present in the air at all.IntensiveandExtensivePropertiesThermodynamic properties can be divided into two general classes, intensive and extensiveproperties. An intensive property is independent of the amount of mass. The value of anextensive property varies directly with the mass. Thus, if a quantity of matter in a given stateis divided into two equal parts, each part will have the same value of intensive property as theoriginal and half the value of the extensive property. Temperature, pressure, specific volume,and density are examples of intensive properties. Mass and total volume are examples ofextensive properties.HT-01 Page 4 Rev. 0